In recent years, attention has been paid to a hybrid vehicle (hereinafter referred to as “HEV”) equipped with a hybrid system having an engine and a motor generator that are compositely controlled according to an operation state of a vehicle in view of fuel efficiency improvement, environmental measures, and the like. With this HEV, a driving force is assisted by the motor generator when the vehicle accelerates or starts, while a battery is charged by regenerative power generation by the motor generator when the vehicle travels inertially or brakes (for example, see PTL 1).
In general, the engine and a transmission in the HEV are connected via a wet clutch. The wet clutch is connected/disconnected during shifting gears in a transmission.